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PL0_SSP can always be derived _and_ it isn't actually even >>> needed at context switch time, only when running userspace. >> True but, just to understand, what is the performance like if you >> actually use XSAVES/XRSTORS to update SSP0 on context switches instead >> of WRMSR+cache?  Hopefully the microcode does a bunch of optimizations >> as well. > First, we can use WRMSRNS for this MSR, so it's faster than usual. > Second, on kernel=>kernel context switches that don't go out to > userspace, it doesn't need to be written at all. It's kinda like the > user FPU state. Third, I'm actually reusing the fred_ssp0 cache, so > further reduces the incremental cost. XSTATE_CET_S is a weird thing.  It contains MSRs for PL{0..2}_SSP which are the shstk equivalents of tss->rsp{0..2}. The Ring1/2 aspects are practically useless; CET-SS was the thing which finally caused me to put Ring1 PV kernels to rest.  All of tss->rsp{0..2} are switched together, so I suppose PL{0..2}_SSP were grouped for the same reason, not that 64bit OSes use task switching. Windows uses %xmm in the main ABI, so already need to XSAVE/XRSTOR when switching between kernel threads.  So it's "simple" to add one extra bit to RFBM while doing this. VMMs probably want to use XSTATE_CET_S simply to have the guest state held together.  Even then, it does feel like a waste; PL1/2_SSP are going to be 0 ~all of the time, while PL0_SSP is nonzero ~all of the time in a CET-enabled kernel, so you can't even rely on the INIT optimisation to speed things up. Linux obviously doesn't want to do an X{SAVE,RSTOR}S between kernel threads, and isn't using Rings 1/2, so the WRMSRNS approach seems fine.  And I agree that it only needs to be good at the point of returning to userspace. > Fourth, it's not _really_ > context-switched. There's never a need to read the register. It can > always be conjured out of thin air. The requirements are different, depending on FRED or not. Without FRED, PL0_SSP must point to a Supervisor Shadow Stack token.  IRET marks the token non-busy (squashing any errors), and IDT delivery checks it can atomically set the token busy, yielding #CP if not. With FRED, ERETU checks that SSP == SL0_SSP and yields #CP on a mismatch.  There are no checks on entry. ~Andrew